1. Field of the Invention
The present invention relates generally to digital cordless telephones and, more particularly, to a new and improved apparatus for directly sequencing an RF carrier with a code sequence for transmission and then directly converting the RF carrier to baseband upon reception without adding low frequency or DC noise.
2. Description of Related Art
There are numerous cordless phones that use analog frequency modulation in the 46/49 MHZ band. Such analog cordless phones have become very popular. Nonetheless, they suffer from range limitations, few channels, and minimal security. As a result, there is a need to provide a cordless phone product that provides a longer range without dropouts, more channels, and greater security.
Use of digital modulation and digital coding techniques offers more robust voice communication over a radio channel, although requiring greater channel bandwidth. Digital modulation also has a capture effect that greatly surpasses co-channel and adjacent channel interference, thereby providing a more noise-free conversation. Use of digital modulation encoding also allows for the addition of effective scrambling codes to greatly improve telephone security. In addition, by using the industrial, medical, and scientific (ISM) band for radio transmission and reception (902-928 MHZ), increased power levels above those in the 46/49 MHZ band are permitted, thus increasing the operating range. The primary FCC requirement for operating in the ISM band at the highest transmit power levels is using direct sequence spread spectrum (DSSS) or frequency hopping spread spectrum (FHSS) modulation.
DSSS modulation provides bandwidth spreading that is large compared to the bandwidth required by the information signal. A DSSS system uses a series of "chips" from a very fast code sequence for spreading an RF carrier, often by modulating the carrier using binary phase shift keying (BPSK). The receiver, of course, must duplicate the code sequence to "despread" the received signal. In order to remove the code sequence, a DSSS system generally samples the received analog signal with an A/D converter and then passes the digital signals through a matched filter at the sampling rate. The conventional DSSS system uses ordinary, 3-sample wide .vertline.E.vertline.-.vertline.L.vertline. time tracking and standard demodulation wherein the correlation "on time" peak is used for demodulation and the sample before and after the peak is used to perform a time discrimination function .vertline.E.vertline.-.vertline.L.vertline. to allow a timing NCO lock and to remove small "on time" peak timing errors. Such systems are usually called E, OT, L systems.
A hypothetical DSSS system of lowest cost and power consumption would use single sample per chip A/D conversion and matched filtering. The present inventor is unaware of any DSSS systems, however, that does this because of time tracking and demodulation problems when sampling below the Nyquist rate.
The known DSSS systems that use 3-sample wide .vertline.E.vertline.-.vertline.L.vertline. time tracking and standard demodulation typically run multiple samples per chip through the A/D conversion and matched filtering stages to effectively discriminate the correct timing. Such a brute force approach increases design complexity and fabrication cost by requiring more hardware to accommodate the multiple samples and lowers performance by requiring more compute time. The foregoing problem is particularly evident in systems that must process long code sequences for high processing gain.
The conventional DSSS modulation system is, moreover, subject to multipath fading even with multisample per chip processing because the one peak the .vertline.E.vertline.-.vertline.L.vertline. tracking system locks on may not be the best signal.
Conventional direct sequence spread spectrum telephones are typically more expensive than necessary because they use heterodyne (dual conversion) radio techniques. An architecture that offers a cost advantage is Direct Conversion Radio. A Direct Conversion architecture, however, has an inherent DC component problem due to Local Oscillator leakage that typically requires DC offset compensation in the modem.
Conventional direct sequence spread spectrum systems often use codes that are selected for certain desired characteristics. Typical codes sequences include the so-called maximal codes and Gold codes. Such codes, however, are not suitable for a direct conversion architecture because they are not even. The numbers of 1's in a maximal code sequence, for example, is always different than the number of 0's by one chip.